Studies were undertaken to investigate specific immunologic responses by human peripheral blood leukocytes from normal individuals and patients with immunologic deficiency states. Methods were developed to measure specific cytotoxic T-cell responses and humoral antibody responses by B-cells in vitro. Patients with immunodeficiency diseases were heterogeneous with regard to their ability to generate cytotoxic T-cells in vitro. Most patients with hypogammaglobulinemia produced immune cytotoxic T-cells normally, while patients with the Wiskott-Aldrich syndrome and ataxia telangiectasia produced almost no immune cytotoxic T-cells. Defects in the production of immune cytotoxic T-cells may contribute to the increased incidence of neoplasia observed in immunodeficiency diseases.